The ratio of radiogenic argon 40 to potassium supplies information on the source or genesis of components of recent sediments. This is the initial investigation in a larger study on the history of illitic materials using dating techniques. In samples from the deltaic deposits of the Mississippi River, age values for the dominantly silt-sized material of the delta averaged 280 m.y., with little horizontal or vertical variation. The clay-size fractions of the same samples averaged only 166 m.y. This age is lower than one would expect of detritus from Paleozoic shales in the central and eastern part of the source area but may be due to mixtures of younger mixed-layer illite-montmorillonite released from the Cretaceous and Tertiary sections to the west.

The Mississippi River sediments in the delta are commonly gray. A reddish-brown layer found at depth in drill holes, believed to have been deposited by the Red River, showed ages of 690 m.y. for the dominantly silt-sized whole samples and 650 m.y. for the clay fractions. This supports a Red River source for this layer and suggests a common source for the two fractions. One would expect the rapidly deposited deltaic sediments from both the Mississippi and Red rivers to lack any appreciable authigenic K phases, if such exist, because these require slow accumulation well offshore.

Measured ages of the clay fractions in bottom sediments from the Rappahannock River in Virginia are similar to those from the crystalline rocks of the source regions, indicating again that the K phases are ancient detritus.

Clay soils forming over ancient shales can contain illite of the same age as the underlying shales, thus indicating a source for detrital, ancient 1Md illite.

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